The present invention relates to an objective lens system (hereinafter referred to as “objective”), in particular for a projection apparatus used in microlithography, with at least a first group of lenses or parts of lenses made of a first crystalline material and at least a second group of lenses or parts of lenses made of a second crystalline material.
Objectives of the aforementioned kind are known for example from the German patent application DE 199 29 701 A1 by the applicant of the present invention. In one embodiment, it is proposed to use calcium fluoride in parallel with barium fluoride as lens materials. In that particular embodiment, calcium fluoride takes the place of crown glass and barium fluoride takes the place of flint glass to achieve the desired achromatic lens properties.
Projection objectives in the same generic category with lenses made of two different fluoride crystals are also known from the German patent application DE 199 39 088 A1. Fluoride crystals are chosen in these cases because of their comparatively high transmissivity for wavelengths shorter than 200 nanometers. Due to the different Abbé numbers of the fluoride crystals used in the lenses, it is possible to achieve a chromatic correction of the projected image.
In the aforementioned references, the birefringent properties of the crystals are of no concern in the optical design of the objectives.
According to a concept known from U.S. Pat. No. 6,201,634, in the production of fluoride crystal lenses, the lens axes should ideally be aligned orthogonal to the {111} planes of the fluoride crystals in order to minimize stress-induced birefringence. However, in proposing this concept, the aforementioned U.S. Patent implicitly assumes that fluoride crystals are not intrinsically birefringent.
However, as described in the Internet publication “Preliminary Determination of an Intrinsic Birefringence in CaF2” by John H. Burnett, Eric L. Shirley, and Zachary H. Levine of the National Institute of Standards and Technology (NIST), Gaithersburg, Md. (posted on May 7, 2001), single crystal ingots of calcium fluoride also exhibit birefringence that is not stress-induced, i.e., intrinsic birefringence. According to the measurements presented in that study, a light ray traveling in the <110> direction of the calcium fluoride crystal is subject to a birefringence that amounts to. 6.5±0.4 nm/cm at a wavelength of λ=156.1 nm, to 3.6±0.2 nm/cm at a wavelength of λ=193.09 nm, and to 1.2±0.1 nm/cm at a wavelength of λ=253.65 nm. On the other hand, if the light propagation is oriented in the <100> direction or in the <111> direction of the crystal, no intrinsic birefringence occurs in calcium fluoride, as is also predicted by theory. Thus, the intrinsic birefringence has a strong directional dependence and increases significantly for shorter wavelengths.
Measurements made by the applicant have confirmed the intrinsic birefringence of calcium fluoride as reported by the NIST researchers, except for the wavelength of λ=156.1 nm, where a birefringence of 11 nm/cm was measured for a ray propagating in the <110> direction of the crystal.
The indices for the crystallographic directions will hereinafter be bracketed between the symbols “<” and “>”, and the indices for the crystallographic planes will be bracketed between the symbols “{” and “}”. The crystallographic directions are perpendicular to the correspondingly indexed crystallographic planes. For example, the crystallographic direction <100> is perpendicular to the crystallographic plane {100}. Crystals with a cubic lattice structure, which includes fluoride crystals, have the principal crystallographic directions <110>, <{overscore (1)}10>, <1{overscore (1)}0>, <{overscore (11)}0>, <101>, <10{overscore (1)}>, <{overscore (1)}01>, <{overscore (1)}0{overscore (1)}>, <011>, <0{overscore (1)}1>, <01{overscore (1)}>, <0{overscore (11)}>, <111>, <{overscore (111)}>, <{overscore (11)}1>, <{overscore (1)}1{overscore (1)}>, <1{overscore (11)}>, <{overscore (1)}11>, <1{overscore (1)}1>, <11{overscore (1)}>, <100>, <010>, <001>, <{overscore (1)}00>, <0{overscore (1)}0>, and <00{overscore (1)}>. Because of the symmetries of cubic crystals, the principal crystallographic directions <100>, <010>, <001>, <{overscore (1)}00>, <0{overscore (1)}0>, and <00{overscore (1)}> are equivalent to each other. Therefore, those crystallographic directions that are oriented along one of the principal directions <100>, <010>, <001>, <{overscore (1)}00>, <0{overscore (1)}0>, and <00{overscore (1)}> will hereinafter be identified by the prefix “(100)-”, and crystallographic planes that are perpendicular to these directions will also be identified by the same prefix “(100)-”. Furthermore, the principal directions <110>, <{overscore (1)}10>, <1{overscore (1)}0>, <{overscore (11)}0>, <101>, <10{overscore (1)}>, <{overscore (1)}01>, <{overscore (1)}0{overscore (1)}>, <011>, <0{overscore (1)}1>, <01{overscore (1)}, and <0{overscore (11)}> are likewise equivalent to each other. Therefore, those crystallographic directions that are oriented along one of the latter group of principal directions will hereinafter be identified by the prefix “(110)-”, and crystallographic planes that are perpendicular to these directions will also be identified by the same prefix “(110)-”. Finally, the principal directions <111>, <{overscore (111)}>, <{overscore (11)}1>, <{overscore (1)}1{overscore (1)}>, <1{overscore (11)}>, <{overscore (1)}11>, <1{overscore (1)}1>, <11{overscore (1)}> are also equivalent to each other. Therefore, those crystallographic directions that are oriented along one of the latter group of principal directions will hereinafter be identified by the prefix “(111)-”, and crystallographic planes that are perpendicular to these directions will also be identified by the same prefix “(111)-”. Any statements made hereinafter in regard to one of the aforementioned principal crystallographic directions should be understood to be equally applicable to the equivalent principal crystallographic directions.
The concept of rotating the orientation of lens elements in order to compensate for the effects of birefringence is described in the not pre-published patent application DE 101 23 725.1, “Projektionsbelichtungsanlage der Mikrolithographie, Optisches System und Herstellverfahren” (Projection Apparatus for Microlithography, Optical System and Manufacturing Method), of the applicant of the present invention, and also in the not pre-published patent application DE 101 27 320.7, “Objektiv mit Fluorid-Kristall-Linsen” (Objective with Lenses Consisting of Crystalline Fluoride) of the applicant of the present invention.